1. Field of the Invention
The present invention relates to a liquid crystal display for a television, display and the like and its fabrication method. Specifically, the present invention relates to a liquid crystal display containing a vertical alignment liquid crystal with large viewing angle and its fabrication method.
2. Description of the Related Art
The liquid crystal display contains the liquid crystal inserted between a pair of substrates. Each of the pair of substrates has an electrode and an alignment film. A TN (Twisted Nematic) mode liquid crystal display widely used in the past contains the liquid crystal having a horizontal alignment film and positive dielectric anisotropy, and when no voltage is applied, the liquid crystal is aligned substantially parallel to the horizontal alignment film. When voltage is applied, the liquid crystal arises in the direction substantially perpendicular to the horizontal alignment film.
Although the TN mode liquid crystal display has advantages such as a capability of miniaturization, the TN mode liquid crystal display has disadvantages that, first, the viewing angle is narrow and, second, contrast is low. As a method to improve the first disadvantage and to obtain the larger viewing angle, there is an alignment division. According to the alignment division, a single pixel is divided into two areas and the liquid crystal is made to arise and lie down to one direction in one area while the liquid crystal is made to arise and lie down to the other direction in the other area, thereby forming the areas with different viewing angle characteristics within a single pixel. When observed as a whole, the viewing angle characteristics are leveled and a larger viewing angle is obtained.
In order to control the alignment of the liquid crystal, rubbing is usually performed on the alignment film. When domain dividing is performed, one area of the single pixel is rubbed in a first direction by using a mask and the other area of the single pixel is rubbed in a second direction which is the opposite direction from the first direction by using a complementary mask. As another way, the whole alignment film may be rubbed in the first direction and ultraviolet irradiation is selectively performed in one area or in the other area of the single pixel by using the mask, thereby creating a difference in pre-tilt in the liquid crystal between in one area and the other area.
Since the liquid crystal display using the horizontal alignment film requires rubbing, damages generated by contamination or static electricity occurring during the rubbing process is a main cause of reduction in yield.
On the other hand, in a VA (vertically Aligned) mode liquid crystal display using the vertical alignment film, when no voltage is applied, the liquid crystal is aligned substantially perpendicular to the vertical alignment film and when voltage is applied, the liquid crystal lies down in the horizontal direction to the vertical alignment film. In this way, high contrast is obtained and the low contrast, which is the second disadvantage of the TN mode liquid crystal display above, is eliminated. However, rubbing is also normally performed on the alignment film to control the alignment of the liquid crystal in the general VA mode liquid crystal display using the vertical alignment film.
The Japanese Patent Application No. 10-185836 by the applicant of this application proposes a liquid crystal display which can control the alignment of the liquid crystal without rubbing. This liquid crystal display is a VA mode liquid crystal display having the vertical alignment film and the liquid crystal with negative dielectric anisotropy and has a linear structure (a protrusion or a slit) arranged on each of the pair of substrates in order to control the alignment of the liquid crystal.
It will be noted, hereinafter, the VA mode liquid crystal display according to this method is referred to as an MVA (Multi-domain vertical Alignment) liquid crystal display in this application.
This MVA liquid crystal display has an advantage that rubbing is not required and, further, the domain dividing is achieved by the arrangement of the linear structure. Therefore, this MVA liquid crystal display can obtain a wide viewing angle and high contrast. Since rubbing is not required, a fabrication of the liquid crystal display is simple, the contamination to the alignment film during the rubbing process is eliminated, and reliability of the liquid crystal display is improved.
FIG. 32 is a diagram of a basic structure of the MVA liquid crystal display, showing a single pixel and its periphery. Further, throughout the diagram, items assigned the same reference numeral indicate the same thing and their repeated description is omitted.
An MVA liquid crystal display 130 is an active matrix type liquid crystal display having a thin film transistor (hereinafter, referred to as a TFT) 14 at each pixel as a switching device, and there are a red pixel R, a green pixel G and a blue pixel B in the pixel to perform the color display.
On a TFT substrate where the TFT 14 is provided, a gate bus line 10 partially serving also as a gate electrode of the TFT 14 and a drain bus line 12 are formed. The TFT 14 consists of a drain electrode 12D extending from the drain bus line 12, a source electrode 12S positioned facing the drain electrode 12D, and an overlapping portion between the drain electrode 12D and the source electrode 12S of the gate bus line 10. Further, although not shown, a channel layer made of, for example, amorphous silicon (a-Si) is formed on the gate bus line. Furthermore, a pixel electrode 16 connected to the source electrode 12S is formed on the TFT substrate. In the pixel electrode 16, a slit 18 is provided diagonally to the pixel and this slit 18 becomes a structure to control the alignment of the liquid crystal on the substrate side. A connecting portion 16a is provided at the pixel electrode 16 so that the pixel electrode 16 is not electrically separated by the slit 18. In this way, a pixel electrode 16 in a pixel is electrically connected.
Although not shown, on a color filter substrate (hereinafter, referred to as a CF substrate) where a color filter is formed, a protrusion 20 to control the alignment of the liquid crystal on the CF substrate side is formed and controls the alignment of the liquid crystal together with the slit 18 on the TFT substrate.
For example, when a diagonal distance of an XGA LCD (liquid crystal display) panel is equal to 15 inches, the size of a single pixel is equal to 99 μm×297 μm, the widths of the slit 18 and the protrusion 20 are equal to 10 μm each, the distance between the slit 18 and the protrusion 20 is 25 μm. Further, the width of the connecting portion 16a of the pixel electrode 16 is equal to 4 μm and the distance between an end portion of the drain bus line 12 and an end portion of the pixel electrode 16 is equal to 7 μm.
FIGS. 33a, 33b and 33c are simplified cross sectional views at a line I-I in FIG. 32 and shows actions of the slit 18 and the protrusion 20 which are the structures to control the alignment of the liquid crystal.
FIG. 33a shows a state of the liquid crystal when no voltage is applied between the electrodes on a pair of substrates. The pixel electrode 16 is formed on a glass electrode 24 on the TFT substrate side, and the slit 18 is formed on the pixel electrode 16. Further, an alignment film (vertical alignment film) 32 is formed covering the pixel electrode 16 and the slit 18. On the other hand, a common electrode 26 is formed on a whole surface of a glass substrate 22, facing the pixel electrode 16, and the protrusion 20 made of an insulator (a dielectric) such as photoresist is formed on the common electrode 26. Further, an alignment film (vertical alignment film) 28 is formed covering the common electrode 26 and the protrusion 20.
Furthermore, a liquid crystal layer LC is in between the TFT substrate and the CF substrate, and liquid crystal molecules (indicated by ellipses in the diagram) are aligned perpendicular to the alignment films 32 and 28. Therefore, the liquid crystal molecules are also aligned perpendicular to the alignment film 28 formed on the surface of the protrusion 20, and the liquid crystal molecules adjacent to the surface of the protrusion 20 are in an inclined state against the glass substrate 22. However, when closely observed, the liquid crystal molecules adjacent to the surface of the protrusion 20 are not aligned perpendicular to the alignment film 28, because the liquid crystal molecules are aligned substantially perpendicular to the glass substrate 22 by the alignment film 28 in an area where the protrusion 20 is not formed and due to the continuum characteristics of the liquid crystal, the liquid crystal molecules follow the liquid crystal molecules occupying a portion of the pixel and are in a state inclined from the direction perpendicular to the alignment film 28 to the direction of a normal line of the glass substrate. Also, although not shown, a pair of polarizing plates are arranged on the outside of the glass substrates 22 and 24 in the state of cross-Nicol. Therefore, in a state where no voltage is applied, the display becomes a black display.
FIG. 33b shows equipotential lines when voltage is applied between the electrodes on a pair of substrates and FIG. 33c shows the state of the liquid crystal in the case above. As shown by equipotential lines shown by dotted lines in FIG. 33b, when voltage is applied between the electrodes 16 and 26, distribution of an electric field in the portion where the slit 18 and the protrusion 20 are formed becomes different from the other portion. This is because in the portion where the slit 18 is formed, an oblique electric field is formed from the end portion of the electrode toward the opposing electrode, and in the portion where the protrusion 20 is formed, the electric field is distorted, since the protrusion 20 is a dielectric provided on the electrode 26. Therefore, as shown in FIG. 33c, the liquid crystal molecules lie toward the direction of the arrow in the diagram. In other words, the liquid molecules lie toward the direction perpendicular to the direction of the electric field depending on the magnitude of the voltage, thereby providing a white display in a state when voltage is applied. At this time, when the protrusion 20 is arranged linearly as shown in FIG. 32, the liquid crystal molecules adjacent to the protrusion 20, having the protrusion 20 as the boundary, lie to two substantially perpendicular directions to the direction where the protrusion 20 is arranged. Since the liquid crystal molecules adjacent to the protrusion 20 are slightly inclined toward the perpendicular direction to the substrate even when no voltage is applied, the liquid crystal molecules adjacent to the protrusion 20 quickly respond to the electric field and lie down, followed by surrounding liquid crystal molecules which also lie down quickly and are influenced by the electric field. In a similar manner, when the slit 18 is provided linearly as shown in FIG. 32, the liquid crystal molecules adjacent the slit 18, having the slit 18 as a boundary, also lie to two substantially perpendicular directions to the direction where the slit 18 is arranged.
Thus, in the area between the two alternate long and short dash lines in FIG. 33a, the liquid crystal molecules fall down to the same direction. In other words, the area aligned in the same direction is formed. This area is indicated by [A] in FIG. 32. As shown representatively by [A] through [D] in FIG. 32, since areas aligned to four different directions are formed in a single pixel, in the MVA liquid crystal display 130, characteristics of wide viewing angle can be obtained. It will be noted that alignment control can not only be performed when the slit 18 and the protrusion 20 are combined as shown in FIGS. 32, 33a, 33b and 33c, but also when a protrusion and a protrusion or a slit and a slit, as a structure to control the alignment, are combined.
However, although wide viewing angle can be obtained in the MVA liquid crystal display 130, an area of liquid crystal molecules that are not stable exists, and therefore, the problem of reduction in brightness exists. In other words, when voltage is applied between the electrodes, an alignment defect area 40 shown by hatching in FIG. 32 occurs. Since this alignment defect area 40 is an area where the transmissivity of the light is poor, the alignment defect area results in a reduction in brightness when the white display is performed. When viewed in the plane of FIG. 32, this alignment defect area 40 occurs on the side where the structures (protrusion of slit) provided on the CF substrate form an obtuse angle with an edge portion of the pixel electrode 16. This occurrence of the alignment defect area 40 is caused by a lateral electric field and the like generated by an influence of the drain bus line 12 at the edge portion of the pixel electrode 16. In the area where this alignment defect area 40 occurs, the liquid crystal molecules lie in the different alignment direction from the alignment direction controlled by the structures (the slit 18 and the protrusion 20 in FIG. 32) provided on a pair of substrates. In other words, the alignment of the liquid crystal molecules is disturbed in this area due to the occurrence of the lateral electric field and the like, thereby resulting in a deterioration in display characteristic of the MVA liquid crystal display 130.
In order to solve a problem (occurrence of an alignment defect area) characteristic to this MVA liquid crystal display, the applicant of this application proposes a new structure to reduce influences from a lateral electric field and the like.
FIG. 34 shows an MVA liquid crystal display 140 according to the proposal. A distinctive feature of this structure is that an auxiliary protrusion 20c extending from the protrusion 20 provided in the CF substrate side along an end portion of the pixel electrode 16 where the alignment defect area 40 occurs is provided. The auxiliary protrusion 20c can certainly be formed by the same material and the same process as the protrusion 20 or can be formed separately.
FIGS. 35a and 35b are diagrams describing the auxiliary protrusion 20c to be formed on the CF substrate. As a structure of the CF substrate, as shown in FIG. 35a, a method to form a black matrix BM to be formed on the CF substrate by overlaying color resins forming a color filter is proposed. This method is achieved by forming red resin R, green resin G and blue resin B on the glass substrate 22, and overlapping, as blue resin B with green resin G, blue resin B with red resin R, and red resin R with green resin G, at each end portion. The overlapped portion is the black matrix BM. Then, the common electrode 26 and the like are formed above.
In the case of the CF substrate formed by such a method (hereinafter, referred to as a resin overlaying BM method), a level difference equal to approximately 0.2-1.5 μm occurs at portions indicated by circles in FIG. 35a, in other words, at the portions color resins are overlapped. If there is the level difference like this, an electric line of force concentrates at the portions, thereby causing alignment defects of liquid crystal molecules.
FIG. 35b shows a state when the auxiliary protrusion 20c is formed at the portion of the level difference of the black matrix. The auxiliary protrusion 20c is formed to cover the portion where there is the level difference. In such a state, the height d1 of the level difference is equal to approximately 0.2-1.5 μm as described above, and the height from the peak portion of the auxiliary protrusion 20c is equal to approximately 1.0-2.0 μm. The auxiliary protrusion 20c functions to rotatably align the liquid crystal molecules by easing the inclination at the portion where there is the level difference and not to concentrate the electric line of force by forming a material with low dielectric constant at the portion where there is the level difference at an angular portion. For example, a relative dielectric constant ∈ of the liquid crystal is approximately 6-8 and the relative dielectric constant ∈ of the protrusion material is approximately 3-4.
However, in the portions designated by the circles in FIG. 35b, there is a case when the auxiliary protrusion 20c does not sufficiently cover angular portion due to irregularities of level difference, irregularities of locations forming the protrusion, irregularities of the protrusion shape and the like.
FIGS. 36a, 36b, 37a, 37b and 37c are diagrams showing the problems in the past. In FIG. 36a, a case when the auxiliary protrusion 20c is provided on the CF substrate of the resin overlaying BM method is shown. FIG. 36a shows a cross section at a line I-I in FIG. 34. On the TFT substrate, the drain bus line 12 is formed on the glass substrate 24, the drain bus line 12 is covered with an insulation film 30, and the pixel electrode 1S further formed on the insulation film 30. The insulation film 30 consists of a TFT gate insulation film, a protection film covering the TFT and the like. Hitherto, the width d1 of the auxiliary protrusion 20c is equal to approximately 10 μm, the width d2 where the auxiliary protrusion 20c and the pixel electrode 16 overlap is designed to be approximately 4 μm.
However, if the auxiliary protrusion 20c is formed on the CF substrate of the resin overlaying BM method with this design value, the thickness of the protrusion material becomes thin at the angular portion of a color resin, for example, in the angular portion of the green resin G. Since the common electrode 26 is formed on the surface of the green resin G of the angular portion, the electric line of force heading outwards from the display area concentrates, and the liquid crystal molecules become a state of alignment defect due to the electric field in this portion. Since the area of the alignment defect enters inside the display domain, the similar dark portion as the alignment defect area 40 in FIG. 32 is formed.
Further, besides the resin overlaying BM method described above, there is a method using a black resin as the black matrix (hereinafter, referred to as a resin BM method). According to this resin BM method, the black resin is placed in the area forming the black matrix and each resin is formed in an opening portion (display domain) so that the end portion of each resin overlaps the black resin. Therefore, as is the case in the resin overlaying BM method, the level difference is formed and the similar problem described above occurs.
FIG. 36b shows a case when other color filter shape is applied on the CF substrate, in which a chrome shading film 34 is formed as the black matrix and the color filter is formed on the shading film 34 by patterning the color resin. In this case, the width d1 of the auxiliary protrusion 20c is also equal to approximately 10 μm and the width d2 where the auxiliary protrusion 20c and the pixel electrode 16 overlap is also designed to be approximately 4 μm. As shown in FIG. 36b, when formed according to the design value, the concentration of electric lines of force heading outwards from the display area is suppressed, the alignment of the liquid crystal molecules is stabilized and the display becomes favorable. However, at the stage when a product is actually fabricated, various irregularities during the fabrication occur, and in many cases, desired characteristics are not obtained.
FIGS. 37a, 37b and 37c are diagrams showing problems of misalignment in lamination and shot unevenness as irregularities during the fabrication. FIG. 37a shows a case in which a misalignment occurs when the CF substrate and the TFT substrate are laminated, and the width d1 of the auxiliary protrusion 20c is equal to approximately 10 μm as is the case in FIG. 36b. However, in FIG. 37a, the TFT substrate is deviated in an upper right direction to the CF substrate in the diagram, thereby resulting in the width d2 where the auxiliary protrusion 20c and the pixel electrode 16 overlap by approximately 3 μm. Therefore, the control power to the liquid crystal molecules is weakened and, an influence from the lateral electric field caused by the drain bus line at the end portion of the pixel electrode 16 occurs, therefore the alignment defect area as indicated by the hatched portion in the diagram occurs. However, in the case of FIG. 37a, the alignment defect area is under the auxiliary protrusion 20c and does not affect the display. It will be noted that when a misalignment in lamination occurs, the opposing width becomes wider at one end portion of the corresponding pixel electrode and the width becomes narrow at the other end portion. That is, in order to have a sufficient opposing width at the corresponding end portions, a margin for laminating extremely reduces and fabrication also becomes difficult.
At this time, as shown in FIG. 37c, exposure and the like are performed by dividing the display domain of a single panel into a plurality of divided areas SA-SD . . . when fabricating the liquid crystal display (liquid crystal panel). Therefore, the same display characteristics can be obtained within each divided area SA-SD . . . However, when there are deviations or the like during exposure, display characteristics may be different from other divided areas.
FIG. 37b shows other divided area in the same panel as FIG. 37a and shows a case when shot irregularities occur during exposure. In FIG. 37b, since irregularities occur when patterning the pixel electrode 16, although the distance d3 from the end face of the color resin B to the end face of the pixel electrode is supposed to be equal to 7 μm according to the original design value as shown in FIG. 37a, the distance d5 in FIG. 37b is equal to 7.5 μm. Therefore, the width d4 where the auxiliary protrusion 20c and the pixel electrode 16 overlap becomes to be equal to 2.5 μm and the alignment defect area indicated by the hatched portion occurs. Furthermore, the alignment defect area appears in the display domain instead of being hidden by the auxiliary protrusion 20c. Therefore, in FIG. 37b, the alignment defect area 40 as shown in FIG. 32 occurs in the display domain.
In FIG. 37c, if the divided area SA is an area where the pixel electrode 16 is formed in the position in accordance with the design standard as shown in FIG. 37a and the divided area SB is an area where the position of the pixel electrode 16 is deviated from the predetermined position due to shot irregularities as shown in FIG. 37b, although a desired bright display is performed in the divided area SA when a certain display is performed, the alignment defect area occurs in the divided area SB, thereby resulting in a dark display. In other words, an irregular shot phenomenon occurs.
FIG. 38 is a diagram showing the relationship between the design value of the overlapping width (opposing width) and the generation ratio of shot irregularities in which the overlapping width between the auxiliary protrusion and the pixel electrode. Here, an attention should be paid that the design value of the overlapping width on the lateral axis is not the overlapping width inside the actual panel. Even if the panel is fabricated according to a certain design value, irregularities of several μm due to a misalignment of lamination between an upper and lower substrates, inaccuracy of the pattern for the structure (protrusion, color resin for color filter, etc.) formed on the substrate or the influence from the divided areas described above occur inside the actually fabricated panel. Therefore, the value of the overlapping width, when the whole display domain is viewed, is in a certain range. In this case, the alignment defect area appears inside the display domain in the portion where the overlapping width is small and the difference in brightness partially occurs in the whole display domain. In such a case, shot irregularities are considered to have occurred at the design value.
When observing this, if the design value of the overlapping width, in other words, if the design center is equal to approximately 4 μm, the irregularity in shooting occurs at a ratio of substantially 50%. The range of the values for the actual overlapping widths in this case is considered to vary from approximately 1 μm to 7 μm. If the design center is equal to approximately 6 μm, the shot irregularities are almost eliminated. The overlapping width in this case is considered to vary from approximately 3 μm to 9 μm.
Thus, in the MVA liquid crystal display in the past, as is the case using the color filter of the resin overlaying BM method or the resin BM method, a problem that many display defects in which brightness reduces occurs when there is a large level difference on the substrate. Further, there is a problem that poor yield caused by extremely small margin in fabrication exists as is the case because a display defect is easily caused by slight irregularities in fabrication.
Therefore, an object of the present invention is to provide a liquid crystal display which is high in brightness and has preferable display characteristics and its fabrication method.
Another object of the present invention is to provide a liquid crystal display with large margin in fabrication, high yield, and preferable display characteristics and its fabrication method.